One-step hologram (including holographic stereogram) production technology has been used to satisfactorily record holograms without the traditional step of creating preliminary holograms. Both computer image holograms and non-computer image holograms may be produced by such one-step technology. In some one-step systems, computer processed images of objects or computer models of objects allow the respective system to build a hologram from a number of contiguous, small, elemental pieces known as elemental holograms or hogels. To record each hogel on holographic recording material, an object beam is typically directed through the a spatial light modulator (SLM) displaying a rendered image and interfered with by a reference beam. Examples of techniques for one-step hologram production can be found in the U.S. Pat. No. 6,330,088 entitled “Method and Apparatus for Recording One-Step, Full-Color, Full-Parallax, Holographic Stereograms,” and naming Michael A. Klug, Mark E. Holzbach, and Alejandro J. Ferdman as inventors, which is hereby incorporated by reference herein in its entirety.
In many holographic recording systems, and particularly in one-step reflection holographic recording systems, a diffuser is used to evenly distribute light in the object beam on to the holographic recording material. For example, a vertical diffusing element (VDE) can be used to spread light vertically in order to increase the vertical viewzone size (e.g., increase the vertical viewing angle) for horizontal-parallax-only (HPO) holographic stereograms. Typically, the diffuser is an anisotropic diffuser. The VDE's function is to provide an anisotropic diffusion plane on which the horizontal image components are focused. The function of the VDE can be accomplished with a lenticular lens array, an interferometric holographic diffuser, a diffractive grating, a specifically designed holographic optical element (HOE) or a combination of these.
Typically, the VDE is placed in close proximity to the holographic recording material (e.g., holographic film) during exposure in order to locate the vertical focus of the hologram either on the hologram plane or as close as possible to the hologram plane. The VDE can also be image relayed to the hologram plane with an appropriate object beam lens system. Generally, it is more effective to physically place the VDE in contact or nearly in contact with the holographic recording material since this tends to provide larger viewing angles. The close proximity of the VDE to the holographic recording material also reduces artifacts that may arise due to low frequency speckle interference between neighboring diffusion elements. FIG. 1A illustrates the situation where the holographic recording material is placed close to the VDE. In this example where the VDE is a lenticular array 100, the VDE is located so that foci of the lenticules (110) making up lenticular array 100 coincide with holographic recording material 120. Thus, if the VDE is placed close enough to the hologram plane, the speckle artifacts are minimized because the light propagating through the VDE element forms a hologram before it can interfere with light from a neighboring VDE element. FIG. 1B illustrates the situation where the VDE is located further from the holographic recording material, thereby giving rise to the aforementioned image artifacts. As seen from detail 130, overlapping light from adjacent lenticules creates an area of interference leading to spurious gratings and low frequency speckle.
In recording a reflection hologram, the reference beam and the object beam are directed at the holographic recording material from the opposite sides of the material. Because of the proximity of the diffuser to the holographic recording material and the relative transparency of the holographic recording material, the reference beam passes through the holographic recording material and impinges upon the surface of the diffuser. Thus, placement of the VDE in close proximity to the holographic recording material exposes the VDE to reference beam light that is transmitted through the film from the side opposite of that to which the reference beam is directed. This situation is illustrated in FIG. 2. During hologram recording, the reference beam light 200 is reflected off the VDE elements (typically at a variety of angles) 210 and is recorded as unwanted noise gratings 220 in holographic recording material 120. Thus, the surface of diffuser 100 typically reflects light from the reference beam back through the holographic recording material a second time.
The reflected light from the reference beam can be reflected such that it interferes with the reference beam as it traverses the holographic recording material. Light from the reference beam passes through the holographic recording material and is reflected by the VDE as reflected reference beam portions. An interference pattern corresponding to the reflected light is recorded in the holographic recording material, resulting in an undesirable artifact that resembles a vertical line seemingly positioned infinitely deep with respect to the hologram plane. This results from the recording of a single beam hologram of the diffuser surface. This artifact is both distracting to the viewer of the resulting hologram and damaging to the diffraction efficiency of the holographic recording material, thereby effecting brightness of the image. Additionally, reflected light from the reference beam can be reflected such that it interferes with the object beam, potentially creating additional unplanned interference patterns that are recorded in the holographic recording material. While in principle, those recorded interference patterns are similar to the interference patterns that are intended to be recorded (i.e., the interference pattern created by the original, un-reflected, reference beam and the object beam), the fact that the interference patterns were formed using light reflected from the reference beam means that additional distortion or unwanted artifacts may be present.
A number of strategies have been used to reduce and/or eliminate the problem of interaction between the reference beam and the diffuser. One solution is to place an anti-reflection coating on the diffuser surface. However, anti-reflective coatings usually are effective only for particular bandwidths of wavelengths and certain angles of incidence of incoming light. Due to the extreme and varied angles at which a reference beam may strike a diffuser and due to the fact that some diffusers are volumetric devices that have no surface relief, this technique has not proven successful. In practice, anti-reflective coatings typically eliminate only about 30% of reflected reference beam light, whereas to eliminate the artifacts described above a greater percentage of the reflected reference beam light should be eliminated. Furthermore, anti-reflective coatings are difficult to uniformly apply over large areas such as the surface area of a diffuser, can be fragile, and can be very costly.
Another technique is the use of a light control or “louver screen” film between the diffuser and the associated holographic recording material. As illustrated in FIG. 3A, light from the reference beam passes through the holographic recording material and impinges upon louver screen film 300, where the light is absorbed, and/or generally prevented from reflecting back toward the reference beam by microlouvers within the film. The object beam (not shown) passes through the VDE and, because of the structure of the louvers 310, generally passes through the louver screen film. Louver screen film is a commercially available (e.g., Light Control Film from 3M™) volumetric substrate that typically contains microscopic opaque strips or louvers, arranged in a parallel formation at a selected variable angle analogous to a venetian blind arrangement. Louver screen film is chosen with a particular louver spacing and angle that allows passage of the object beam light, for example, at angles of zero to plus or minus thirty degrees (±30°), while absorbing reference beam light incident at higher angles of, for example, approximately forty five degrees (45°). Such louver screen film successfully prevents reference beam light from striking and reflecting off the surface of diffuser 100, and thus eliminates the unwanted artifacts.
One problem associated with using louver screen film is the film's requisite thickness (on the order of 1 mm) which necessarily further separates the diffuser from the surface of the holographic recording material. FIG. 3B illustrates a situation where the thickness of louver film 300 limits the amount of diffused object beam light that arrives at the holographic recording material. For example, while ray 320 successfully traverses louver film 300, ray 330 does not. Because the louver screen film separates the diffuser and holographic recording material, the diffuser plane and the hologram plane are not as close together as is possible, which leads to poorer quality recorded holograms. Louver screen film may also introduce other artifacts into the hologram due to the film's periodicity and diffractive effects associated with the passage of light through the narrow louvers of the film. Additionally, it can be difficult to match the pitch of the louver film with the pitch of the lenticules in VDE 100, and to properly register the two devices. Finally, louver film often absorbs a significant percentage of the object beam light, again due to the existence of louvers within the film material, along with intrinsic substrate and surface absorption and reflection.
Yet another solution is to use a specially designed holographic optical element, in place of the louver film, that diffracts the unwanted reference beam light away from the holographic recording material. Examples of such devices can be found in the U.S. Pat. No. 6,369,920 entitled “Reference Beam Deflecting Element for Recording a Hologram,” naming Michael A. Klug as the inventor, which is hereby incorporated by reference herein in its entirety.
Nevertheless, it is desirable to have new devices to reduce or prevent reflections of the reference beam off the VDE from striking the holographic recording material. Such devices overcome the deficiencies of the prior art, including for example, the thickness, efficiency, and ease of construction and use.